Place the mushrooms in a colander, add the salt, stir thoroughly and let sit for 3 hours. Now wash under cold water and let dry. Heat half the oil in a saucepan over a low heat, add the ginger and cook for 5–7 minutes until golden and caramelised. Add the garlic and chilli, lower the heat and cook for 5 minutes more, stirring frequently, then return the heat to medium, add the mushrooms and cook until they start to break down, about 7 minutes. Add the beans and cook for a further 5 minutes, then add the fish sauce, soy, sugar and rice vinegar. Bring to the boil, reduce the heat to a low simmer and cook for 12 minutes until everything is nicely combined. Remove from the heat and leave to cool, then stir in the remaining oil. Transfer to a sterilised jar or container and place in the fridge until needed (it will keep for ages and I reckon it tastes better the longer you leave it).